The invention relates to a support material for offset-printing plates in the form of a sheet, a foil or a web, comprising pretreated aluminum or an alloy thereof and having, on at least one surface, a hydrophilic coating of a polymer containing acidic side groups. The invention also relates to a process for the production of a support material and to a printing plate comprising the support material.
Support materials for offset-printing plates are provided, on one or both sides, with a photosensitive layer (reproduction layer), which is applied either directly by the user or by the manufacturers of precoated printing plates. With the aid of this layer a printing image is produced by a photomechanical route. Following the production of the printing image, the layer support comprises the image areas which print and, simultaneously, the hydrophilic image background required for the lithographic printing process is formed in the areas which are free from an image (non-image areas).
Thus, a layer support for a photosensitive material used for the production of lithographic plates must meet the following requirements. First, those portions of the photosensitive layer which have become comparatively more soluble following exposure must be capable of being easily removed from the support by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue. The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water, rapidly and permanently, during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink. The photosensitive layer must also exhibit an adequate degree of adhesion prior to exposure, and those portions of the layer which print must exhibit adequate adhesion following exposure.
Base materials which can be used for layer supports of this kind include aluminum, steel, copper, brass or zinc foils, but also plastic sheets or paper. By appropriate processing operations, such as, for example, graining, matte chromium-plating, surface oxidation and/or application of an intermediate layer, these raw materials are converted into layer supports for offset-printing plates. The surface of aluminum, which is presently the most frequently used base material for offset-printing plates, is roughened according to known methods, e.g. dry-brushing, slurry-brushing, sandblasting, chemical and/or electrochemical treatment, or combinations of these treatments. In order to increase the resistance to abrasion, the roughened substrate may additionally be treated in an anodizing step to produce a thin oxide layer.
In practice, the support materials, and particularly anodically oxidized aluminum support materials, are often subjected to a further treatment step, before applying a photosensitive layer, in order to improve the adhesion of the layer, increase the hydrophilic properties and/or improve the developability of the photosensitive layers. Such treatments can be carried out according to known methods.
For example, DE-C-907 147 (=U.S. Pat. No. 2,714,066), DE-B-14 71 707 (=U.S. Pat. No. 3,181,461 and U.S. Pat. No. 3,280,734) or DE-A-25 32 769 (=U.S. Pat. No. 3,902,976) describe processes for hydrophilizing support materials for printing plates, comprising aluminum which has optionally been anodically oxidized. In these processes, the materials are treated, with or without the application of an electric current, with an aqueous solution of sodium silicate.
DE-A-11 34 093 (=U.S. Pat. No. 3,276,868) and DE-C-16 21 478 (=U.S. Pat. No. 4,153,461) describe the use of polyvinylphosphonic acid or of copolymers based on vinylphosphonic acid, acrylic acid and vinyl acetate to hydrophilize support materials for printing plates, comprising aluminum which has optionally been anodically oxidized. The use of salts of these compounds is also mentioned, but is not specified in detail,
According to DE-B-13 00 415 (=U.S. Pat. No. 3,440,050) complex fluorides of titanium, zirconium or hafnium are used to produce an additional hydrophilization of aluminum oxide layers on support materials for printing plates.
Apart from these hydrophilizing methods, which have become known in particular, numerous polymers have been described for use in this field of application. For example, DE-B-10 56 931 describes water-soluble, linear copolymers on a basis of alkyl vinyl ethers and maleic anhydrides which are used in photosensitive layers for printing plates. Of these copolymers those are particularly hydrophilic, in which the maleic anhydride component has not been reacted or has been more or less completely reacted with ammonia, an alkali metal hydroxide or an alcohol.
As disclosed in DE-B-10 91 433, support materials for printing plates comprising metals are hydrophilized with film-forming organic polymers, for example, with polymethacrylic acid or sodium carboxymethylcellulose or sodium hydroxyethylcellulose, in the case of aluminum supports or with a copolymer of methyl vinyl ether and maleic anhydride, in the case of magnesium supports.
According to DE-B-11 73 917 (=UK 907,718) support materials for printing plates comprising metals are hydrophilized by means of polyfunctional amino/urea/aldehyde resins or sulfonated urea/aldehyde resins which are initially water-soluble and are cured to a water-insoluble state on the metal support.
DE-B-12 00 847 (=U.S. Pat. No. 3,232,783) describes a hydrophilic layer which is prepared on a support material for printing plates by coating the support first with a) an aqueous dispersion of a modified urea/formaldehyde resin, an alkylated methylolmelamine resins or a melamine/formaldehyde/polyalkylenepolyamine resin, then with b) an aqueous dispersion of a polyhydroxy or polycarboxy compound, such as sodium carboxymethylcellulose, and the substrate coated in this manner is finally treated with c) an aqueous solution of a Zr, Hf, Ti or Th salt.
DE-B-12 57 170 (=U.S. Pat. No. 2,991,204) describes a hydrophilizing agent for support materials for printing plates, comprising a copolymer which contains not only acrylic acid, acrylate, acrylamide or methacrylamide units, but also Si-trisubstituted vinylsilane units.
DE-A-14 71 706 (=U.S. Pat. No. 3,298,852) discloses the use of polyacrylic acid as a hydrophilizing agent for support materials for printing plates made of aluminum, copper or zinc.
The hydrophilic layer on a support material for printing plates described in DE-C-21 07 901 (=U.S. Pat. No. 3,733,200) is formed of a water-insoluble hydrophilic acrylate or methacrylate homopolymer or copolymer having a water absorption of at least 20% by weight.
DE-B-23 05 231 (=U.S. Pat. No. 1,414,575) describes a process for hydrophilizing support materials for printing plates, in which a solution or dispersion comprising a mixture of an aldehyde and a synthetic polyacrylamide is applied to the support.
DE-A-23 08 196 (=U.S. Pat. No. 3,861,917) discloses hydrophilization of grained and anodically oxidized aluminum supports for printing plates, using ethylene/maleic anhydride or methyl vinylether/maleic anhydride copolymers, polyacrylic acid, carboxymethylcellulose, sodium poly(vinylbenzene-2,4-disulfonic acid) or polyacrylamide.
DE-B-23 64 177 (=U.S. Pat. No. 3,860,426) describes a hydrophilic subbing layer for offset-printing plates of aluminum, which is disposed between the anodically oxidized surface of the printing plate support and the photosensitive layer and contains a cellulose ether and, additionally, a water-soluble Zn, Ca, Mg, Ba, Sr, Co or Mn salt. The cellulose ether is contained in the hydrophilic subbing layer in a layer weight of 0.2 to 1.1 mg/dm.sup.2, the same layer weight is specified for the water-soluble salts. The mixture of cellulose ether and salt is coated on the support in the form of an aqueous solution employinq, if appropriate, an additional organic solvent and/or a surfactant.
To consolidate anodically oxidized aluminum surfaces, U.S. Pat. No. 3,672,966 describes aqueous solutions of acrylic acid, polyacrylic acid, polymethacrylic acid, polmaleic acid or copolymers of maleic acid with ethylene or vinyl alcohol, which are applied after sealing the surfaces, in order to prevent seal coats.
The hydrophilizing agents used for printing plate support materials according to U.S. Pat. No. 4,049,746 contain saline reaction products obtained from water-soluble polyacrylic resins containing carboxyl groups and polyalkylenimine/urea/aldehyde resins.
UK 1,246,696 describes hydrophilic colloids, such as hydroxyethylcellulose, polyacrylamide, polyethylene oxide, polyvinylpyrrolidone, starch or gum arabic for use as hydrophilizing agents on anodically oxidized aluminum supports for printing plates.
EP-B-0 149 490 describes compounds containing amino groups and, in addition, carboxyl or carboxylate groups, sulfo groups or hydroxyl groups, which are used for a hydrophilizing treatment. However, this publication starts out from monomers and specifies a molecular weight of 1000 as an upper limit.
For hydrophilizing support materials for printing plates the prior art has also disclosed the use of those metal complexes which have low-molecular weight ligands. These include, for example: complex ions comprising divalent or polyvalent metal cations and ligands including ammonia, water, ethylenediamine, nitrogen oxide, urea or ethylenediaminetetraacetate, according to DE-A-28 07 396 (=U.S. Pat. No. 4,208,212); iron cyanide complexes, such as K.sub.4 (Fe(CN).sub.6) or Na.sub.3 (Fe(CN).sub.6), in the presence of heteropolyacids, such as phosphomolybdic acid or the salts thereof or of phosphates, according to U.S. Pat. No. 3,769,043 and/or U.S. Pat. No. 4,420,549, and iron cyanide complexes in the presence of phosphates and complexing agents, such as ethylenediaminetetraacetic acid, for use in electrophotographic printing plates having a zinc oxide surface, according to U.S. Pat. No. 3,672,885.
EP-A-0 069 320 (=U.S. Pat. No. 4,427,765) describes a process, in which the salts of polyvinylphosphonic acids, polyvinylsulfonic acids, polyvinylmethyl-phosphinic acids and other polyvinyl compounds are used as post-treating agents.
DE-A-26 15 075 (=UK 1,495,895) describes a process for treating image-carrying offset-printing plates, which uses polyacrylamide or a mixture of polyacrylamide and polyacrylic acid.
SU-A-647 142 teaches the use of a copolymer of acrylamide and vinyl monomers for hydrophilizing offset-printing plates.
DE-C-10 91 433 describes a process for post-treating supports for offset-printing plates using polymers of methacrylic acid, methyl vinyl ether and maleic anhydride.
Acrylamide for use in the treatment of printing plate supports is also mentioned in DE-A-25 40 561.
To the same end, in particular to improve the storability of printing plates, DE-A-29 47 708 describes, among others, Ni salt solutions of acrylamide and acrylic acid as well as of acrylamide and vinylpyrrolidone.
The above-described methods, however, have more or less serious disadvantages, which means that the support materials so prepared often no longer meet the requirements which must now be met in offset printing in view of developer resistance, water/ink balance, roll-up characteristics and print run stability. Thus, for example, after treating support surfaces with alkali metal silicates which produce a good developability and good hydrophilic properties, a certain deterioration of the storability of photosensitive layers applied to these surfaces must be accepted and the print run of a printing plate post-treated in this manner is drastically lowered.
Although the complexes of the transition metals basically enhance the hydrophilicity of anodically oxidized aluminum surfaces, they have, nevertheless, the disadvantage of being very readily soluble in water, such that they can be easily removed upon developing the layer with aqueous developer systems which lately contain increasing proportions of surfactants and/or chelate formers which have a high affinity for these metals. As consequence, the concentration of the transition-metal complexes on the support surface is more or less strongly reduced, which may also reduce the hydrophilic action.
When supports are treated with water-soluble polymers, without having a possibility of anchoring these polymers, the good solubility of the latter, in particular in aqueous-alkaline developers which are predominantly used for developing positive-working photosensitive layers, will also lead to a marked decrease in the hydrophilizing effect.
Monomeric, hydrophilic compounds, as described, for example, in EP-B-0 149 490, generally have the disadvantage that during the developing and printing processes, they are relatively quickly washed away from the bared surface in the non-image areas and lose their hydrophilizing action, since an insufficient number of anchoring positions are present in the surface.
Even combining a mixture of a water-soluble polymer, such as cellulose ether, and a water-soluble metal salt, leads to reduced adhesion of the reproduction layer, because the layer weights and thus the layer thicknesses used are relatively high (see DE-B-23 64 177). Reduced layer adhesion may, for example, manifest itself by the fact that, in the developing process, portions of the developer liquid penetrate under image areas.